The present invention relates to a fastener assembly and more specifically to a preassembled fastener unit engageable with an internally threaded element, such as a nut to provide a clamped joint for one or more workpieces. The fastener unit of the present invention is of the general type comprised of a screw threaded member having a frusto-conical washer mounted thereon in captive, assembled relation by the external thread form.
Before discussing in detail the present invention, it is believed that a brief review of the prior art as to the structural features and inherent deficiencies thereof will be beneficial. It is only after one recognizes what is demanded in the art and what has been tried in order to fulfill this demand that the present invention can be viewed in its proper perspective and appreciated. In this regard, preassembled fastener units, such as discussed above, viz, a washer captively assembled to a screw threaded member, have long been known and used in the art. In point of fact, many industries, and primarily the automobile industry, specify use of preassembled fastener units in the numerous installations or joints that must be effected in the manufacture of their products. It should be noted, that the fastener units being referred to are of the type which employ a frusto-conical washer, the use of which serves to maintain a tight joint.
In manufacturing these preassembled fastener units of the prior art type, an unthreaded, headed blank is provided and the frusto-conical washer member positioned thereon, adjacent the head portion. The fastener blank is then subjected to a thread forming process, such as thread rolling wherein a screw thread is formed on a portion thereof. This type of threading procedure produces, in effect, an upsetting of the blank material and a re-working thereof to produce a desired thread form, and must be distinguished from a machining operation wherein the blank material is removed to provide the thread form. When a thread is produced by a cold-working process, the major dimension of the thread form produced, i.e., the crest diameter, is necessarily greater than the original cross-sectional dimension of the unthreaded blank portion, while the minor thread dimension, i.e., the root diameter, is less than the original cross-sectional dimension of the unthreaded blank portion. Accordingly, by sizing the washer aperture so that it will freely pass over the unthreaded blank portion and subsequently forming the thread as discussed above, the resulting major thread diameter will be greater than the aperture dimension and thus will serve to maintain the apertured washer in assembled relation on the screw shank.
It will be appreciated from the above, that the thread forming takes place with the washer assembled on the shank, adjacent the headed portion thereof. Therefore, where a frusto-conical washer is employed, the external thread thus formed can only extend along the shank to a location coincident with a plane defined by the rim of the washer. That is to say, the frusto-conical washer prevents the thread rolling dies from extending into the concavity of the washer.
Consideration is now directed to some of the problems inherent with the above-mentioned prior art structures, which are overcome by the present invention as will be discussed more completely hereinafter. At numerous installations in an automobile, the fastener units of the type discussed above are prescribed by design engineers for engagement with a nut to clamp one or more workpieces. Frusto-conical washers are specified in many instances, such that they can be compressed during the formation of the joint to establish residual forces which will maintain a relatively tight joint. These residual forces are obtained only due to the elastic deformation or deflection of the washer upon assembly. Therefore, should said deflection not be obtained, there is a danger that the joint will come loose in service.
In a joint, wherein a relatively thin workpiece or pieces are clamped between the fastener unit and the nut element, as is frequently the case in automobile constructions, a distinct problem exists with the prior art type of construction. In this regard, the nut must be advanced toward the driving head a sufficient distance to effect the desired compression or deflection of the frusto-conical washer. However, the extent of travel of the nut is limited by the external thread length, in that the internal thread of the nut will engage the unthreaded portion of the fastener shank in interfering engagement and preclude any further advancement of said nut. With relatively thin workpieces this often will occur before the desired degree of deflection or compression of the washer can be obtained. As such, the joint may become loose in service, and considering automobiles, for example, will generally produce a rattle. Needless to say, when a rattle results, it is the manufacturer's obligation, through its dealer, to correct this rattle which necessitates the costly expenditure of considerable effort and time.
Again, with a view toward placing the present invention in proper perspective, the above-noted and discussed problems with the prior art type of fastener unit shown in the drawing has been recognized by those skilled in this art, and numerous solutions have been proposed, as will be discussed hereinafter. Insofar as it is known, all of the prior art solutions involve an attempt to extend the external thread into the concavity of the washer, which is accomplished by assembly of the frusto-conical washer to the screw member after formation of the external thread.
More specifically, with the approach taken by the prior art devices, the washer member is provided with central aperture structures which allow it to be engaged over the external thread form and then the material of the washer body deformed or staked to reduce the effective minimum dimension of said aperture to a value of less than the major diameter of the external thread. The washer is thus captively assembled on the screw member and since thread forming takes place prior to assembly of the washer, the screw thread may be extended inwardly of the rim of the conical portion.
The fastener units of the general type discussed in the preceding paragraph are of numerous and varying designs. For example, the washer may be provided with one or more tabs about the central aperture, which tabs are initially deflected outwardly so as to permit the washer to pass over the external thread, and are then deformed inwardly to produce the captive relation. Or alternately, the washer may be initially formed with a flared neck portion which defines the central aperture, which is subsequently deflected or staked inwardly to reduce the effective diameter of said aperture and produce the desired, preassembled unit.
It should be kept in mind that all of these prior art approaches involve an additional assembly operation, subsequent to the thread forming step, which subsequent operation in and of itself is an inherent disadvantage from the standpoint of production costs. More importantly, these prior art constructions are possessed of certain inherent structural disadvantages which can materially affect their reliability and performance in service. Specifically, with the prior art method of subsequently assembling the washer, the washer first must be subjected to an initial forming operation to provide the desired structural configuration adjacent the aperture, and then deformed a second time to effect the preassembled engagement with the threaded screw member. This cold-working of the washer material often produces stress cracks or zones of cold worked, brittle material that could subsequently result in cracks.
These cracks often migrate to the exterior of the washer and thus result in a complete fracture. When fracture occurs, the washer is incapable of functioning to the intended degree and the joint may fail or a rattle result.
Thus, while the above-discussed prior art devices and methods amount to a distinct step forward solving the basic problems inherent with the fastener of the type shown in FIG. 1, and provide a measure of satisfactory performance, there still existed room for improvement. The present invention serves materially to fill the void left by these devices and provides numerous advantages over the prior art concepts in both ease of manufacture, cost and dependability.